1. Technical Field
The invention relates to anti-lock brake systems for motor vehicles and more particularly to an exciter ring and disc brake rotor assembly which allows snap in fitting and retention of the exciter ring on the disc brake rotor.
2. Description of the Problem
Brake units for motor vehicles should provide smooth braking with reasonable service life. In some applications this need has been met with disc brakes. The cost effectiveness of disc brake systems depends in part on making the components out of easily worked, inexpensive material. Disc brake rotors have generally been made from relatively inexpensive gray iron castings. Gray iron is, however; highly susceptible to corrosive attack, particularly in the operating environment of vehicles where brake components are open to the air, subject to substantial transient heating and exposed to water and salt water spray. In regular use, the working surfaces of the discs are rubbed clean by contact with the disc pads, which are typically made of a composite material and which rub off corroded areas. However, other areas of the brake discs are not swept by the brake pads and are not regularly cleaned. Prior to anti-lock braking systems, such concerns were not paramount with brakes which were frequently in use, since the rotor is a regularly replaced part and the remaining areas subject to attack were not critical.
With the advent of anti-lock braking systems (ABS) other sections of the disc brake rotor take on importance, particularly the anti-lock brake system exciter ring. The exciter ring is a cylindrical section of the rotor having a common axis of rotation with the rotor. A plurality of teeth are formed in a ring, which is flat in the plane of rotation of the rotor to pass closely by a stationary sensor. One type of sensor used is a variable reluctance sensor which generates an electrical pulse train as a function of the varying magnetic flux leakage between the sensor head and the exciter ring. In this system the frequency of the resulting electrical pulse train indicates the rotational speed of the wheel on which the rotor is mounted. The generation of clean pulse train is greatly aided by having teeth of uniform shape, size and spacing. Where the ring is cast as one piece with the rotor, corrosion of the rotor can compromise all of these factors, resulting in difficulty in detecting the passage of teeth and gaps and causing generation of an irregular pulse train.
The problem of corrosion of exciter rings for anti-lock brake systems is not limited to disc brake systems, but is also an issue with drum brakes. In drum brakes the exciter ring has not been an integral part of the drum, but rather has been a separate part, press fitted on the end of a wheel hub. Press fitted parts can readily be made of material more corrosion resistant than gray iron, such as a mild low carbon steel. However, the press fitting of rings onto disc rotors has proven less successful than it has for hubs using drum brakes. The difficulty stems from the fact that in disc brake systems the exciter ring is in direct contact with the rotor, which is part of the active brake assembly, whereas the axle hub using drum brakes is not a brake component. On drum systems less heat is transferred from the hub which carries the exciter ring than is transferred in disc systems from the rotor to an exciter ring. Exciter rings are made of low carbon steel which has a different thermal coefficient of expansion than does iron. The difference in coefficients of expansion in the materials used for the ring and the hub or rotor causes more problems in disc brake systems than in drum systems since more heat is transferred by a rotor to a ring than by a hub to a ring and thus an exciter ring and rotor vary more in size in relation to each other than do an exciter ring and a hub. An exciter ring which loses its tight fit with a rotor can begin to clock (i.e., the ring rotates relative to the rotor). If a ring rotates on a rotor, the ring will not reflect actual wheel rotational velocity. This affects ABS operation. Such a ring could also “pop” off.